Generally, in order to remove impurities, such as hydrogen sulfide, mercaptan (R—SH), organic acid and the like, included in the product generated from an oil refining process and a petrochemical process, the product makes contact with caustic soda (NaOH) or is scrubbed. Caustic soda (aqueous solution), which is a typical strong base, exists in a white translucent crystal state, has deliquescence absorbing moisture in air, and is used in the form of an aqueous solution in which caustic soda is dissolved in water. Particularly, caustic soda is steadily used as a primary reactant for removing impurities because it is effective at removing impurities as well as it is safe and economically efficient.
However, caustic soda having been used in removing impurities is converted into spent caustic soda. Spent caustic soda must be suitably treated because it is harmful to the human body. In particular, a spent caustic soda solution is difficult to be directly treated by a general biological waste-water disposal method because it has high pH, remarkably high biochemical oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC) and the like.
Several methods have been proposed in order to treat such spent caustic soda. Among these methods, an incineration method is generally used. In the incineration method, the liquid components of a spent caustic soda solution are evaporated by burning fuel oil or the like to produce carbon dioxide, alkali metal carbonate and the like, and harmful components are removed by a disposal facility such as incinerator or the like and are then discharged to the atmosphere or the water system. However, this incineration method is disadvantageous in that its operation cost is high, and it causes air pollution during incineration.
Further, a wet air oxidation method is also known. In the wet air oxidation method, spent caustic soda is treated by bringing spent caustic soda into oxygen gas microbubbles to cause an oxidation reaction and then diffusing the condensed reaction product using bulk liquid. In this case, an organic material is converted into carbon dioxide and water, and an inorganic material (sulfide) is converted into thiosulfate or sulfate. However, the wet air oxidation method is problematic in that high investment cost is required because it needs a preheater, an oxidation reactor, a cooler and a separator.
Besides, when acidic oils (cresol-based oil, naphthenic base oil, etc.) included in the spent caustic soda discharged from an oil refining process and/or a petrochemical process are to be treated by the wet air oxidation method, reaction conditions (temperature and pressure) must be minutely controlled, and, particularly, high temperature and/or high pressure reaction conditions must be created according to the properties thereof, thus increasing cost.
Meanwhile, a Fenton oxidation treatment method using bivalent iron compounds (for example, ferrous sulfate (FeSO4), iron chloride (FeCl2), etc.) and hydrogen peroxide is also known. However, this Fenton oxidation treatment method is problematic in that a large amount of inorganic sludge is generated.
Recently, in oil refining and petrochemical plants, regulations for liquid or gaseous effluents causing air or water pollution have become stricter. Further, spent caustic soda is difficult to treat because it has high BOD and COD, so it is expressly provided in U.S. Resources Conservation and Recovery Act (RCRA) that spent caustic soda is defined as “D003 (reactive sulfide)”.
Like this, spent caustic soda must be properly treated in order to observe the plant emission regulations. That is, a proper treatment method is required to be used in consideration of type and characteristics of spent caustic soda. Particularly, when spent caustic soda contains a comparatively large amount of phenol-based compounds (phenol, cresol, etc.) known as highly-dangerous carcinogenic substances, more careful attention is required.
Accordingly, a spent caustic soda treatment process, which is improved compared to conventional technologies, is required.